Color activeColor

The color that should be used in the active state (i.e., when value is true).

For example, a checkbox should use this color when checked.

Source 

Color get activeColor => _activeColor;
void activeColor=(Color value)

Source 

set activeColor(Color value) {
  assert(value != null);
  if (value == _activeColor)
    return;
  _activeColor = value;
  markNeedsPaint();
}